1. Field of the Invention
This invention relates generally to apparatus for protecting a facility and its loads from surges in supply voltage, and more particularly, to a surge suppression network responsive to a rate of change in supply voltage indicative of a surge to clamp the voltage across the load to a specified voltage level.
2. Background Information
Various devices and arrangements are known for protecting loads from surges in supply voltage. Generally, two approaches are used: Series and parallel protection. In series protection, a high impedance is used in series with the load during a surge to block or limit surge current. In parallel protection, the surge current is diverted with a low impedance shunt. The present invention provides parallel protection.
It is known to use voltage clamping devices and crowbar devices to provide parallel surge protection. The voltage clamping devices clamp the voltage across the load to a specified level. A common type of clamping device is the metal oxide varistor (MOV). If used alone to provide parallel protection, the MOV must have a clamping voltage that is substantially above the nominal supply voltage so that its maximum continuous operating voltage (MCOV) is above the nominal supply voltage. Known crowbar devices include gas discharge tubes and thyristors. These devices normally have a high impedance and switch to a low impedance when a surge in the supply voltage exceeds the breakdown voltage of the gas in the case of the gas discharge tube, or is high enough to activate a trigger circuit to turn the thyristor on. A major difficulty with gas discharge tubes is that they are difficult to turn off even when the voltage goes through zero. An inherent limitation of the crowbar devices when used alone is that the low impedance that they switch into parallel with the load, depresses the load voltage below nominal.
U.S. Pat. No. 4,912,589 suggests a surge suppression network in which a clamping device such as a MOV is connected in series with a crowbar device in the form of a gas tube or a trigger device such as a silicon controlled rectifier (SCR) or a TRIAC. A triggering circuit in parallel with the clamping device switches the crowbar device when a surge exceeds a breakdown or trigger voltage. Thus, this network is responsive to the amplitude of the voltage surge.
U.S. Pat. No. 5,621,599 discloses a parallel protection circuit described as a "switched MOV device." In this network, an MOV is connected in series with a TRIAC across the load. The TRIAC is triggered on by a voltage applied to the gate by a resistor network so that again, the network is voltage responsive.
A limitation of these known protection circuits utilizing crowbar devices, in addition to the difficulty in turning them off, is that they are relatively slow to respond. Also, the known parallel surge suppression networks which respond to voltage are intentionally triggered by harmonics and/or temporary over-voltage disturbances.
There is a need for an improved surge suppression network. This includes a need for a surge suppression network that does not respond to harmonics or temporary over-voltage disturbances.
There is also a need for an improved surge suppression network incorporating MOVs as clamping devices which does not require an MCOV well above the nominal supply voltage.